For example, in Japanese Laid-open Patent Application Publication No. 2001-136706, a motor is disclosed where the axial length of the rotor magnet is set to be longer than the axial length of the stator. In addition, it is disclosed that the magnetic center C2 of the rotor magnet is displaced in the axial direction from the magnetic center C1 of the stator and an attractive force in the axial direction between the magnetic centers C2 and C1 is generated. The attractive force is transmitted to the rotor shaft which is connected to the rotor magnet, and the rotor shaft can be pressed downward (to the thrust plate).
As illustrated in FIG. 8 of Japanese Laid-open Patent Application Publication No. 2013-117300, such a motor is also used in a fan. The fan is frequently used in the household appliances and the like for cooling, and so quietness is required. One of the noise sources of the fan is the motor unit. A motor with high cogging torque and torque ripple cannot rotate smoothly, and results in a generation of vibration and noise. In order to suppress the cogging torque and the torque ripple, it is disclosed to change gradually the surface magnetic flux density in the circumferential direction of the rotor magnet, for example, by making the magnetization waveform similar to a sinusoidal wave (see Japanese Laid-open Patent Application Publications No. 2003-111360 and No. H09-140104).
The present inventors studied the case where the magnetization waveform of the surface magnetic flux density in the circumferential direction of the rotor magnet is a sinusoidal waveform in a motor having the axial length of the rotor magnet longer than the axial length of the stator, and found that the position detection by the Hall sensor of the brushless motor rarely became unstable. However, when a groove was introduced on the outer periphery of the rotor shaft in order to facilitate the assembly of the thrust washer on the rotor shaft which prevent falling out of the rotor shaft and the thrust washer was fitted in the groove, it was observed that the position detection by the Hall sensor became unstable.
The width of the above groove should be wider than the thickness of the thrust washer because it is difficult to perform the assembly if the width of the groove is same to the thickness of the thrust washer. Consequently, the rotor shaft can move in the thrust direction (axial direction) by an amount corresponding to the difference between the width of the groove portion and the thickness of the thrust washer. Then, when the rotor shaft moves in the thrust direction (axial direction), the rotor magnet connected to the rotor shaft also moves in the thrust direction, and the distance between the rotor magnet and the Hall sensor varies. As a result, it becomes difficult for the Hall sensor to detect the magnetic field correctly, and the detection of the position based on the magnetic field becomes unstable.
In such a case, there is an option to use a Hall sensor of high sensitivity capable to detect the magnetic field even at a large distance. However, such a high-sensitivity Hall sensor is expensive, and it is not desirable in terms of cost.
Another option is to provide a stronger hold down force in the thrust direction, in order to suppress the movement of the rotor magnet in the thrust direction. To achieve this purpose, it is necessary to obtain a hold down force in the thrust direction by changing the magnetization waveform of the surface magnetic flux density of the rotor magnet. However, as understood from the disclosures of Japanese Laid-open Patent Application Publications No. 2003-111360 and No. H09-140104, taking into consideration the suppression of the cogging torque and the torque ripple, it is not easy to change the magnetization waveform corresponding to the surface magnetic flux density of the rotor magnet.